Chromatic confocal technique is a well-known technique for tridimensional (3D) surface mapping and thickness measurements, for semiconductor or other industrial applications.
The technique relies on the use of a chromatic lens with an enhanced chromatism, whose focal length depends strongly on the optical wavelength. Each wavelength of the light crossing such lens is focused at a different distance, or in a different focal plane.
The chromatic lens is embedded in a confocal set-up with source and detection apertures placed at confocal planes of the chromatic lens, so as to reject out-of-focus light. When a reflecting interface is placed in front of the chromatic lens, only the light with the wavelength whose focal plane corresponds to the position of the interface is transmitted by the detection aperture.
Detection is made by a spectrometer, which comprises usually a dispersing element and a sensor (CCD or CMOS) to acquire the intensity spectrum of the light. The height (or distance) of the interface relative to the chromatic lens is obtained by analyzing the intensity spectrum of the detected light.
Such set-up allows measuring distances on a single point at the time. So inspecting large surfaces may be very time-consuming.
Acquisition speed can be improved by providing several measurement channels in parallel.
For that, two kind of architectures are known, which are for instance described in the document FR 2 950 441.
It is for instance known to use a bulk beam splitter cube which is common to all the measurement channels. In that case, the light issued from the source apertures crosses the beam splitter and the chromatic lens, and the light reflected by the interfaces is directed by the same beam splitter towards the detection apertures.
This kind of arrangement allows providing a large number of channels, but it has the drawback that it is very difficult to adjust for matching optically the respective source and detection apertures of all the measurement channels. So usually this kind of architectures is implemented with an approximate confocal configuration, using for instance slits.
It is also known to use fiber couplers which direct the light from the source towards the chromatic lens, and the reflected light towards the detectors. Such configuration has the advantage that the source and detection apertures are the same (the end of a measurement fiber), and thus the optical alignment is very easy.
However, the fiber couplers have several drawbacks, notably:                they are difficult to use with a very large number of channels;        due do their principle of operation using coupling of modes between fiber cores, their coupling ratio is very dependent with the wavelength, which may introduce bias in the measurements.        
It is an object of the invention to provide a chromatic confocal device allowing implementation of a large number of channels.
It is also an object of the invention to provide a chromatic confocal device allowing such implementation in a small volume.
It is also an object of the invention to provide a chromatic confocal device with a large number of channels which is easy to build and align.
It is also an object of the invention to provide a chromatic confocal device with optimal optical and metrological characteristics.